This invention relates to a load lifting apparatus, and more particularly to an extendible boom forklift.
An extendible boom forklift generally includes one or more extendible and retractable boom sections with a fork section connected at the distal end of the boom sections and disposed at an angle relative to the boom sections. The boom sections are operable to move a load supported by the fork section toward and away from the vehicle to which the boom sections are mounted. During extension or retraction of the boom section, the fork section moves in a direction generally parallel to the axis of the booms. In most field applications, the axis of the booms is disposed at an angle to the axis of the vehicle frame when it is desired to extend or retract the booms to move a load. Thus, during extension or retraction of the booms, the load supported by the fork section moves at an angle relative to the vehicle frame.
In many field applications, it is desirable to move the load at a constant elevation relative to the vehicle frame when entering, positioning or withdrawing the fork relative to the load. With present boom control systems, it is necessary to simultaneously manipulate two separate control levers to maintain the fork at a constant elevation relative to the vehicle frame during extension or retraction of the booms. That is, while the operator is actuating the boom extension or retraction mechanism, the boom lift mechanism must simultaneously be actuated in order to maintain the altitude of the fork. Such simultaneous manipulation of separate control levers is difficult to accomplish and, even when the operator is skilled, results in jerky fork movements caused by the necessary intermittent actuation of the lift mechanism. When the load is raised, such jerky movements can greatly affect the stability of the machine.
Additionally, with or without a separate level extension mechanism, it is often difficult for an operator to see adequately when the booms are raised so as to even attempt to provide level movement of the load.
It is known to provide a separate mechanism at the fork end of the extendible boom members which provides a limited amount of level traverse of the forks. However, such a mechanism must be designed to carry the rated load and is therefore bulky and heavy. With its location at the distal end of the boom members, this type of mechanism requires substantial counterbalancing in order to prevent tipping of the forklift. It is also known to construct a forklift so that the entire mechanism is movable along rails mounted to the vehicle chassis to provide level traverse of the forks. Such a construction provides a limited amount of level travel, and, by its nature, creates a shifting center of gravity of the machine during such movement. Counterweights or outriggers are generally required to balance the machine.
It is an object of the present invention to alleviate the above-noted problems in extending or retracting the booms of an extendible boom forklift or the like while maintaining the fork elevation during traverse The invention provides a single control which allows the fork section to maintain a constant elevation relative to the frame of the vehicle during extension or retraction of the booms, thus eliminating the need to jockey two separate controls to achieve such level extension or retraction, and provides a control mechanism which is not part of the load carrying mechanism. In accordance with the invention, an extendible boom forklift or the like having one or more extendible boom members mounted thereto, with a fork section or the like connected to the distal boom member and disposed at an angle thereto, is provided with direction controlling means for controlling the direction of movement of a predetermined point on the boom sections, such as the fork section or the like, relative to the vehicle frame during extension and retraction of the boom members. In one embodiment, the direction controlling means is disposed between the vehicle frame and the proximal end of the boom members, and maintains the fork section at a constant elevation relative to the vehicle frame during extension and retraction of the boom members. The direction controlling means includes an upper stationary element, such as an upper shaft, mounted generally parallel to the axis of the boom members at the proximal end of the boom members, and a lower stationary element, such as a lower shaft, mounted substantially parallel to the vehicle frame. A control means is operative between the upper and lower shafts for maintaining the elevation of the distal end of the boom members relative to the vehicle frame during retraction or extension of the boom members. In one embodiment, the control means operative between the upper and lower shafts includes an extendible member slidably mounted at its ends to the upper and lower shafts, with the extendible member including an extension portion and a stationary portion. A clamping means is provided for clamping the extension portion of the extendible member during retraction and extension of the one or more boom members to provide a relatively fixed length for the extendible member. A drive means is provided for moving the extendible member along the upper and lower shafts in a direction substantially parallel to the frame axis during extension or retraction of the boom members in an amount proportional to the extension or retraction of the boom members. In this manner, when the upper shaft is disposed at an angle relative to the lower shaft, the movement of the extendible member along the upper and lower shafts tends to compress or elongate the extendible member. Adjustment means is provided responsive to the tendency of the extendible member to compress or elongate. The adjustment means adjusts the angle between the upper shaft and lower shaft, and therefore between the boom members and the vehicle frame, during extension or retraction of the boom members. Such adjustment of the angle between the boom members and the vehicle frame maintains the fixed length of the extendible member, and thereby the constant elevation of the fork section relative to the vehicle frame during extension or retraction.
A novel mechanism is employed for clamping the extension portion of the extendible member for fixing the length of the extendible member during extension or retraction of the boom members. The clamping mechanism generally comprises a housing mounted to the stationary member and accommodating passage of the extension member therethrough, with a sleeve disposed within the housing and also having a passage therethrough. The sleeve has an inner surface in close proximity with the extension member. The sleeve includes a relatively thin walled portion about at least a portion of the inner surface in close proximity with the extension member. A fluid passage is disposed adjacent the thin walled portion of the sleeve, and is adapted to receive fluid under sufficient pressure to deform the thin walled portion of the sleeve so as to cause the inner surface of the sleeve to frictionally engage the extension member. Such frictional engagement of the extension member by the sleeve prevents relative movement between the stationary member and the extension member. A small amount of movement of the housing is allowed so that, during extension or retraction of the boom members, the housing moves a small amount relative to the stationary member so as to indicate the tendency of the extendible member to elongate or compress during its movement along the upper and lower shafts by the drive means during extension or retraction of the boom members.
The adjustment means includes a novel mechanism for sensing the tendency of the extendible member to compress or elongate by sensing the small amount of movement allowed in the housing during extension or retraction of the boom members, and to adjust the angle between the upper and lower shafts in response to such movement. Broadly speaking, the mechanism translates a linear movement into proportional fluid pressure. The mechanism includes a sensing means for detecting a linear movement, such as that of the housing of the clamping mechanism, and a control means proportionally responsive to the sensing means for adjusting the angle between the upper and lower shafts. The control means generally includes a valve for placement in a hydraulic circuit, and the movements of the housing are compensated for by the valve directing fluid pressure to hydraulic cylinders connected between the vehicle frame and the boom members. The amount of extension of the hydraulic cylinders is adjusted responsive to movement of the housing of the clamping mechanism, to adjust the angle between the boom members and the vehicle frame to maintain the fork section at a constant elevation during extension or retraction of the boom members. In one embodiment, a sensing mechanism is interconnected with the extension portion of the extendible member through the housing of the clamping mechanism, and includes an actuator for actuating the control valve according to the direction and magnitude of movement of the housing portion of the clamping mechanism. The actuator may comprise a pivotable rocker arm sensitive to movements of the housing portion of the clamping mechanism, with end portions of the rocker arm movable toward and away from the control valve according to movements of the housing portion of the clamping mechanism. Plungers provided on the control valve are selectively actuated by the end portions of the rocker arm so as to control retraction and extension of the hydraulic cylinders in response to movement of the extension portion of the extendible member.
In one embodiment, the drive means for moving the extendible member along the upper and lower shafts includes an axially extending toothed rack interconnected with a rotatable gear. The gear rotates responsive to retraction and extension of the boom members in an amount proportional to such extension or retraction. The rack is interconnected with an upper slide provided on the upper shaft, and the extendible member is connected to the upper slide. Movement of the toothed rack thus causes movement of the upper slide and thereby movement of the extendible member which moves the lower slide while maintaining the extendible member 90.degree. to the lower shaft.
A method is also disclosed for controlling the direction of movement of a predetermined point on one or more axially extending segments relative to a base, generally in accordance with the above-discussed features.
The above features of the invention provide an extendible boom forklift which is simple to operate while allowing the operator to maintain the load level and the elevation constant relative to the vehicle frame during retraction or extension of the boom sections. The direction controlling mechanism is relatively simple in theory and operation, and can be easily incorporated into the components of an extendible boom forklift or similar apparatus. An inherent advantage of the invention is that it directs the movement of the load, and is not required to support or carry the load in and of itself. The system does not require counterweights or other such balancing of the machine.